Spree 1-3-stable compatable process_payments! and payment method

Corrects process_payments! ignoring order total when processing entered credit cards. This was causing multiple charges to go through on occasion.

Also replaces references to the Spree::Order payment method since that method was removed from the Spree::Order model in commit:
https://github.com/spree/spree/blob/3d0fa4ff434569ff8491aba690fde87e577ec19c/core/app/models/spree/order.rb

This was causing a method undefined error when spree_store_credits was invoked.